  Citation: Kang, S.; Loader, N.J.; Wang, J.; Qin, C.; Liu, J.; Song, M. Tree-Ring Stable Carbon Isotope as a Proxy for Hydroclimate Variations in Semi-Arid Regions of North-Central China. Forests 2022, 13, 492. https://doi.org/10.3390/f13040492 Academic Editor: Ignacio García-González Received: 29 January 2022 Accepted: 20 March 2022 Published: 22 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article Tree-Ring Stable Carbon Isotope as a Proxy for Hydroclimate Variations in Semi-Arid Regions of North-Central China Shuyuan Kang 1 , Neil J. Loader 2 , Jianglin Wang 1, * , Chun Qin 1 , Jingjing Liu 1 and Miao Song 3 1 Key Laboratory of Desert and Desertification, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China; kangshuyuan@lzb.ac.cn (S.K.); qinchun@lzb.ac.cn (C.Q.); liujj@lzb.ac.cn (J.L.) 2 Department of Geography, Swansea University, Singleton Park, Swansea SA2 8PP, UK; n.j.loader@swansea.ac.uk 3 College of Water Conservancy and Hydropower Engineering, Gansu Agricultural University, Lanzhou 730070, China; songmiao@lzb.ac.cn * Correspondence: wangjianglin2011@lzb.ac.cn Abstract: Carbon and oxygen isotope ratios (δ 13 C and δ 18 O) were measured in annual tree-ring cellulose samples dated from 1756 to 2015 CE. These samples were extracted from Chinese pine (Pinus tabulaeformis Carr.) trees located in a semi-arid region of north-central China. We found that tree-ring δ 13 C and δ 18 O values both recorded similar climatic signals (e.g., temperature and moisture changes), but found that tree-ring δ 13 C exhibited a stronger relationship with mean temperature, precipitation, average relative humidity, self-calibrating Palmer drought severity index (scPDSI), and standard precipitation evaporation index (SPEI) than δ 18 O during the period 1951–2015 CE. The strongest correlation observed was between tree-ring δ 13 C and scPDSI (previous June to current May), which explains ~43% of the variance. The resulting 130-year reconstruction reveals severe drought events in the 1920s and a sustained drying trend since the 1980s. This hydroclimate record based on tree-ring δ 13 C data also reveals similar dry and wet events to other proxy data (i.e., tree-ring width and historical documentation) that have allowed reconstructions to be made across the northern fringe of the Asian summer monsoon region. Our results suggest that both large-scale modes of climate variability (e.g., El Niño-Southern Oscillation, Pacific Decadal Oscillation, and North Atlantic Oscillation) and external forcing (e.g., solar variability) may have modulated moisture variability in this region. Our results imply that the relationship between tree-ring δ 18 O and local climate is less well-characterized when compared to δ 13 C and may be affected more strongly by the influences of these different atmospheric circulation patterns. In this semi-arid region, tree-ring δ 13 C appears to represent a better tool with which to investigate historical moisture changes (scPDSI). Keywords: carbon isotope; oxygen isotope; Chinese pine; hydroclimate; north-central China 1. Introduction Recent changes in the distribution of precipitation along the northern fringe of the Asian summer monsoon (ASM) region in north-central China are believed to have been influenced by the changing intensity of the Asian monsoon and westerly atmospheric circulation patterns in the semi-arid zone [1]. The Ha Si Mountain (HSM) nature reserve is located at the northern edge of the ASM and on the boundary of several semi-arid regions. The HSM is bordered to the north by the Tengger Desert, to the southeast by the Yellow River, and to the west by the north-eastern Tibetan Plateau. This unique geographical position makes the HSM one of China’s most important water conservation regions. The western and southern part of the HSM is influenced by a westerly wind and the ASM system, respectively. Therefore, changes to hydroclimate have important effects on agriculture and the economy in this region. Severe drought events in the past millennium have occurred over different timescales and have had negative socio-economic impacts on Forests 2022, 13, 492. https://doi.org/10.3390/f13040492 https://www.mdpi.com/journal/forests